In a broad sense, "pitch" is any "sticky" substance found in the papermaking process. Sticky substances create problems in the paper making process when they deposit on paper making equipment. More specifically "pitch" refers to any soluble organic matter originating from the extracts of wood including fatty acids and esters, resin acids, and sterols. Pitch may also include process additives such as defoamers, sizing agents, and coatings, as well as inorganic components like calcium carbonate, silica, clay, magnesium and titanium. Pitch is released from wood during chemical and mechanical pulping.
Fine particles of pitch that remain well-dispersed do not create a deposition problem. However, there is a tendency for the hydrophobic pitch particles to agglomerate at the air-water interface. These pitch particles form deposits due to their decreased solubility as they move downstream (through cooler temperatures and lower pHs) and from the increased turbulence of the process. These pitch particles aggregate to form tacky threads or balls which then deposit on paper making equipment including chest walls, screens, paper machine headbox, wires, felts and dryers. This can also lead to sheet holes and breaks in paper resulting in increased downtime and/or lower quality paper.
Pitch deposition can be also be a problem in recycled or secondary fiber processes where organo-soluble pitch contaminants called "stickies", such as hot melts, polyethylene films, latexes, pressure sensitive adhesives, and waxes agglomerate and form deposits. These pitch contaminants deposit on equipment which similarly causes processing difficulties and decreases the quality of the final sheet.
Pitch control agents typically used for years are nonionic surfactants, especially nonyl phenol ethoxylates, talc and alum. Nonionic surfactants, which contain both a hydrophobic and hydrophilic segment, decrease the surface tension, provide wetting, detergency and dispersancy. However, nonyl phenol ethoxylates have a high foaming tendency which may result in higher defoamer usage, thus depositing more pitch. Talc is also used to control pitch deposition by adsorbing colloidal pitch onto the talc particles so it is retained in the sheet. The disadvantage of using talc is that it tends to deposit further downstream especially in paper machine felts and cause felt plugging. In acid paper making systems, alum or aluminum sulfate is used to decrease the tackiness of the pitch by cationic fixation to pulp. When using alum, pH control is critical to prevent deposition of alum.
U.S. Pat. No. 5,266,166 discusses a method for reducing pitch in paper making systems using an effective amount of polyalkylene oxide/vinyl acetate graft copolymer. U.S. Pat. No. 5,292,403 discloses methods for detackifying pitch employing a charged polymer and oppositely charged surface active agent. U.S. Pat. No. 4,964,955 teaches the use of a highly charged cationic polymer adsorbed into a water insoluble substrate thus forming a three dimensional cationic particle that attracts anionic pitch particles and prevents deposition. U.S. Pat. No. 5,230,808 teaches the use of polyethylene oxide in combination with glycol and glycerine as an effective flocculating agent for improving the clarification of waste water in the pulp and paper industry.